This invention relates to the measurement of structures down to and below about 100 nanometers in size by indirect optical methods. More particularly, this invention exploits the interaction of an evanescent electrical field arising from an optical resonator structure with the structure of the measured object. This invention will find immediate application to wafer Critical Dimension metrology in the microelectronics art.
For routine critical dimension (CD) metrology on process wafers, there are at least four possible approaches: (1) far-field optics, (2) scanning electron microscopy (SEM), (3) scanning probe microscopies (SPM) such as near-field scanning optical microscopy (NSOM), and (4) scatterometry.
Each of these techniques have their advantages, as well as their limitations. Optical microscopy is non-invasive, robust, and inexpensive. However, the resolution of a far field optical instrument is Rayleigh limited. At optical wavelengths, sub-micrometer measurements are difficult due to the present computational intractability of the inverse problem. After a slow start in the early 1980's the SEM is now the workhorse of the semiconductor industry for wafer CD metrology. It promises to provide reliable linewidth measurements down to about 250 nm. However, the SEM is an invasive method and requires the inconvenient step of taking the wafer to high vacuum. SPM techniques are conceptually simple and essentially non-invasive. However, the tip convolution problem may prove impossible to overcome. For example, the NSOM tip is typically 250 nm wide (including the aluminum cladding) and must be brought to within about 10 nm of the sample. Therefore, NSOM cannot readily determine linewidths of high aspect ratio structures. Scatterometry appears to have considerable promise at the 250 nm feature size. However, it requires a rather large grating test structure in the scribe-grid and cannot be applied to isolated features.
Simply put, there is an unmet need in the semiconductor wafer processing art to be able to determine the topography of critical dimension structures on the wafer for dimensions at or below 100 nm feature size in a rapid and convenient manner.